Study On Cold Sintering Process Of Different Water-Soluble Lead-Free Piezoelectric Ceramics

In this paper,a new type of ceramic sintering method,cold sintering,and a cold sintering process to prepare piezoelectric ceramic materials for sintering and mass transfer mechanism are studied.Cold sintering was first reported by Clive A in 2016.It refers to a low-temperature liquid phase sintering process achieved by the wettability of a transient solution under a certain temperature and pressure.Although researchers have proposed several possible mechanisms to explain the cold sintering process,there is no clear determination.We have prepared different water-soluble lead-free piezoelectric ceramics by cold sintering process:consistently dissolved sodium nitrite（NaNO₂）,potassium dihydrogen phosphate（KH₂PO₄,KDP）,and slightly water-soluble sodium potassium citrate(K_0.5Na_0.5Nb O₃,KNN)and inconsistent dissolved barium titanate（Ba Ti O₃,BT）lead-free piezoelectric ceramics.It is hoped that it will contribute to the cold sintering mechanism.In this paper,the effects of cold sintering on the phase structure and microstructure of uniformly dissolved Na NO₂and KDP lead-free piezoelectric ceramics were investigated.The XRD results show that the cold sintering process does not change the phase structure of NaNO₂and KDP.The pressure of cold sintering and the sintering time were changed.The change of cold sintering pressure conditions caused the preferred orientation of KDP grains.The cold sintering time had no effect on the density and phase structure of NaNO₂.The experimental results show that the cold sintering process is very suitable for this ceramic material which is uniformly dissolved in water.It provides a hydrothermal environment suitable for crystal growth similar to hydrothermal method,and realizes densification of ceramic materials.For the slightly water-soluble KNN ceramics,the NaCl solution can be used as a transient solution to reduce the sintering temperature of the KNN ceramic.Since the volatilization of alkali metal elements occurs under high temperature conditions,the addition of a transient solution can also serve to supplement the Na element.When the sintering temperature is 900℃,KNN ceramics exhibit a typical orthogonal structure with a Curie temperature T_c=448℃,a piezoelectric constant d₃₃=115 p C/N,and an electromechanical coupling coefficient k_p=32%.Finally,we obtained a ceramic sample with uniform grain size with an average grain size of 0.5μm.The experimental results show that cold sintering is an effective method for sintering KNN ceramics at low temperature.The KNN ceramics prepared by this method have uniform grain size and good electrical properties.Inconsistently dissolved BT ceramics were prepared by cold sintering of Li₂CO₃aqueous solution as a transient solution,and then heat treated at 900℃ to obtain a simple perovskite structure.It is shown that Li⁺completely enters the barium titanate lattice,the sintering temperature is 900℃,and the Curie temperature of the BT ceramic prepared by adding Li₂CO₃to 3 wt% is T_c=138℃,and the piezoelectric constant d₃₃=170 pC/N.The relative dielectric constantε_r=1620,and the dielectric loss tanδ=1%.